I bought mine 1998 legacy outback MT.In 06 with 87000 from first owner.In 07 at 97000 HG gave up(no surprise:cool:).HG done by mine friend.no problem till 175000.When I start notice slightly rough idle,and then miss at #4.Did full tuneup,did't change anything , checked injectors ,and find piece of something(like piece of rubber or old caulking??) in #4.Removed and clean it all was fine,car runes like new.then at 190000,it started with little rough idle again,Did full tuneup.nothing changed .hit higher speed on highway and have the catalytic converter below eff.Changed second o2 sens. cleared the code. And now whenever I hit speed over 80 mph popping code, miss on #4 or#3.and little jerking in traffic at low RPM.Changed fuel filter and fuel pressure regulator changed the SP again NGK. Did the cylinder compression test. Around 180Psi on each.So no problem there.Cables an coil double checked OK .What should be the next step?Vacuum? The first O2 sens?